The optical transmission and/or optical reflectivity of electrochromic coatings can be varied repeatedly and reversibly between low and high transmission states, and/or between low and high reflectivity, by applying an electrical potential between the top surface and bottom surface of the electrochromic coating. In the specialized application to architectural windows, or windows comprising the exterior envelopes of buildings, electrochromic coatings on the window glass can be used to control the amount of sunlight and/or solar heat that enters the building. This control can extend from ultraviolet wavelengths into the infrared, to modulate the entire solar spectrum of sunlight that typically reaches habitable buildings.
In particular, high performance electrochromic coatings must withstand the harsh conditions of intense sunlight and wide variation in temperature that are typically endured by architectural windows, while retaining their ability to cycle between low transmission and high transmission modes, without any degradation in the color or level of transmission for either the low or high transmission mode, over the course of a suitable long service life, say 20 years or about 20,000 cycles between modes.
However, these optical coatings need not be impervious to moisture because they can be isolated from moist air by encapsulating them within a hermetically sealed, double pane, insulating window, where the electrochromic coating resides on one of the interior surfaces of the insulating window, and are only exposed to dry air or inert fill gas.
A specialized subset of electrochromic coatings that is believed to satisfy the service requirements of architectural windows, described above, consists entirely of inorganic metal oxides and nitrides, completely devoid of any carbon containing polymeric materials or silicones. Typically, these coatings make use of ion insertion compounds that can change between low and high (or high and low) optical transmission, over a wide wavelength range. Cathodic materials darken from high to low optical transmission when they are electrochemically reduced—by supplying them with electrons and charge balancing positive ions at a low, reducing electrical potential. Anodic materials darken when they are electrochemically oxidized—by depleting them of electrons and charge balancing positive ions at a high, oxidizing electrical potential.